The present invention relates to stand-off devices to prevent an interior tubular, such as a pipe, casing, or other tubular or rod, from making contact with the sides of a wellbore or an exterior tubular to keep the interior tubular member from becoming stuck, hung up, or damaged. In addition, stand-off devices help facilitate an effective cement job, when cementing an interior tubular inside a well bore or exterior tubular.
In the completion of oil and gas wells, it is standard practice to set or cement at least one string of casing within the wellbore. Casing strings are cemented in the wellbore to prevent fluids from migrating from the production zone through the annulus between the casing string and the wellbore to the surface or other zones where for example fresh water may be contaminated. In addition, there are regulations which require that some zones be cemented off.
In cementing a casing string, a cement slurry is pumped down the interior of the casing string, out the lower end, into the annulus between the string and the wellbore. However, to effect an efficient cementing job, the complete annulus needs to be cemented without pockets in the cement and without areas in which the string is contacting the wall of the wellbore. To facilitate obtaining an effective cementing job the casing is commonly spaced away from the sides of the wellbore with a stand-off device that fits around the casing string. In addition, stand-off devices aide in running the pipe into the hole without hanging up.
Stand-off devices may also be used on tubular members in general, including casing or pipe strings, which are hung within another string of casing or pipe. These inner strings may or may not be cemented within the outer pipe string.
Stand-off devices are not limited in use to oil and gas wells. Stand-off devices may also find utility in any subterranean wells, such as disposal, injection, water, storage, and other types of wells, or any in any application where a tubular is disposed in another tubular or bore hole.
Stand-off devices for casing, tubing or pipe are commonly constructed of low grade carbon steel, and have a tubular body, cylindrical hole, shaft, or sleeve adapted to fit around a pipe joint. These prior art stand-off devices usually include outwardly bowed springs or strips having opposing ends connected to opposite ends of the sleeve. Although the resiliency of the bow strings or strips enables them to move through tight spots in the wellbore, they may not support the weight of the casing string, especially in a highly deviated wellbore.
In another type of prior art stand-off device, the bow strings or strips, which have a gap between the strip and the body of the stand-off device, are replaced by solid blades(no gap). The blades are tapered at each end to provide outer spaced bearing surfaces for engaging the wellbore or the outer casing. Although less prone to collapse than bow springs under the weight of the casing string, prior art blades are not shaped optimally to prevent the stand-off device from becoming stuck in the exterior casing or in the wellbore, as the case may be, upon encountering an obstruction or turn. This is particularly problematic in deviated wellbores, where gravity and other factors can cause the stand-off device to be disposed so off-center around the casing or in the wellbore, or where the standoff device is unable to negotiate around irregularities in the deviated wellbore due to a less than optimal shape of the blades. The stand-off device and the casing may become wedged in the well, and, in any case become unsuitable for providing a suitable cementing job.
Another reason stand-off devices become stuck is the presence of materials in the well that adhere to the stand-off device and inhibit its movement. Such materials include shale, gumbo, sand, and drilling mud.
One reason stand-off devices become stuck or hung up is because of their shape. Prior art blades comprise two portions: a tapered portion at each longitudinal end of each blade, which generally does not, and is not intended to contact the well bore or the outer tubular, and a bearing surface which does contact the inner surface of the bore or outer tubular. Further, prior art stand-off devices comprise a sleeve upon which the blades are disposed. The sleeve is substantially cylindrical, wherein the outer surface curves laterally in accordance with the circular cross section of a tube, but the outer surface does not curve longitudinally. Opposite sides of the sleeve are substantially parallel to each other, longitudinally. The bearing surface of all prior art blades is parallel, longitudinally, to the longitudinal axis of the device sleeve. That is, the bearing surface does not curve longitudinally. This lack of longitudinal curvature in the bearing surface of prior art blades is periodically referred to herein as flatness or being flat. The flatness of the bearing surface of prior art blades exacerbates the tendency of prior art devices to hang up or become stuck.
Prior art stand-off devices have further drawbacks when they are metal, especially when run and set within another string of pipe. One of the drawbacks is when the metal stand-off device is run into or when it vibrates due to slugs in production it contacts the outer pipe string and may cause a spark, which can be very hazardous in a hydrocarbon filled well. Also, metal stand-off devices create a corrosion problem with the casing strings which it contacts through electrolysis. Metal stand-off devices also are susceptible to damage when running acid and circulating the acid back out of the hole. Additionally, there is a concern with scrapping the inner diameter of stainless steel tubing when running stainless/duplex stainless steel tubing having metal stand-off devices and the machined inside diameters of drilling equipment that the stand-off device passes through such as blow out preventors.
It would be a benefit, therefore, to have a stand-off device adapted to fit about a string of pipe for centering the pipe in a wellbore or within an outer string of pipe, wherein the bearing surface of the blades is curved longitudinally to inhibit hang ups and to provide reliable movement of the stand-off device through the wellbore or outer string of pipe.
It would be a benefit to have a stand-off device that prevents materials such as shale, gumbo, sand, and drilling mud from adhering to it.
It would be a further benefit to have a stand-off device that has non-sparking properties.
It would be a still further benefit to have a stand-off device that provides cathodic protection between strings of casing to inhibit electrolysis.
It would be an additional benefit to have a stand-off device that is resistant to deterioration due to acid and caustic substances.
It would be a still further benefit to have a stand-off device that is easily installed either prior to or on location and that is lightweight while still having sufficient strength to withstand the forces encountered in casing operations.